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An alternative, or complement, to obtaining IPD directly from trial investigators is to access IPD through a data repository or data‐sharing platform. In recent years, a number of these have been established to store, curate and share data from completed clinical trials (Box 3.1). Although this is a welcome development and such repositories may have the potential to save time in obtaining IPD, experience of using them in the context of IPD meta‐analysis projects has thus far been limited and mixed. The process of gaining approval to access data can be lengthy, and the approach may not be time saving compared to contacting trial investigators directly.^74,75 As the format and coding of IPD is likely to differ between the trials available through such repositories, and will need to be re‐coded for the IPD meta‐analysis project, the central research team may still need to consult with the original trial investigators, for example to clarify issues, recode variables, and obtain any required information that is missing from the repository materials. Restrictions on how repository data can be used may potentially limit their usefulness for some types of IPD meta‐analysis projects (Sections 3.4 and 4.4.5).

There are also a growing number of repositories that store IPD from studies of relevance to a particular healthcare area or topic (‘topic‐based repositories’). These may be established using the IPD collected for a completed IPD meta‐analysis project, to which additional trials may be added over time. For example, ACCENT is a topic‐based database developed from an IPD meta‐analysis of the efficacy of adjuvant fluorouracil and folinic acid for stage II/III colorectal cancer.⁷⁶ Consequently, an important advantage is that IPD have already been coded to a common format and checked prior to their inclusion in the original IPD meta‐analysis. Although such repositories are often used as a resource for subsequent research carried out by the initial collaborative group, it may also be possible for others to gain access to the repository data. Clearly, if considering using such data from a repository for an IPD meta‐analysis, it is important to establish at the outset whether the repository data will be obtainable.